1. Field of the Invention
The present invention relates, in general, to a novel magneto-optical disk and a novel superdensity disk and, more particularly, to the use of an integral substrate to form a magneto-optical disk or superdensity disk, which has pits and grooves formed on the opposite surfaces, without junction parts, thereby obtaining an improvement in reliability and mechanical properties of the resulting substrate at a reduced cost. Also, the present invention is concerned with a method for manufacturing the magneto-optical disk and the superdensity disk.
2. Description of the Prior Art
As society becomes more and more information-intensive it produces an enormous quantity of various information every day and, thus, recording media with higher data storage density, higher data transfer rate and longer data archival capability are necessary to cope with the information explosion. Further, recording media are required to be of high recording sensitivity with consideration of the margin between drives of different electromagnetic machines. In response to this necessity, magneto-optical recording media were created and, since its creation, have been continuously developed.
Among the optical recording media are a magneto-optical disk (hereinafter referred to as "MOD") and a superdensity disk (hereinafter referred to as "SD"). MOD is capable of recording and reading data by forming a magnetic layer, on a substrate, whereas SD is incapable of rewriting the information stored because they are formed with a stamper during a mastering process, and when making stamper, data is recorded in pits without recording layer.
SD's have recently come into the spotlight in recording media markets not only because they have 10 to 600 times higher recording density as a magnetic recording medium, but also because information can be stored semi-permanently in them by virtue of non-contacting recording/reproducing interaction between a head and the medium.
In order to better understand the background of the invention, a description of conventional MOD and SD will be given below, in connection with some drawings.
Referring to FIG. 5, there is depicted a conventional MOD of double-sided structure that is prepared by bonding two identical subunits to each other using a hot melt bonding layer 50. Each subunit has a polycarbonate substrate 20a and 20b, one side of which is covered with a hard coating protective film 60a and 60b and the other side of which has a tetralayer arrangement. The tetralayer arrangement includes a first dielectric film 21 and 25, a magnetic recording film 22 and 26, a second dielectric film 23 and 27, and a reflective film 24 and 28 protected by a seal coating film 29 is formed.
With reference to FIG. 7, there is depicted a structure of a conventional SD. Like the conventional MOD, the SD has two identical subunits, each having a substrate 71 and 77 on which a semi-transparent film 72 and a reflective film 76 and a seal coating film 73 and 75 are sequentially formed. These subunits are symmetrically arranged with a UV bonding layer 74 interposed therebetween.
Referring to FIG. 3, there is shown a conventional manufacturing process for a substrate of MOD. As shown in this figure, a substrate 2 is manufactured through injection molding. In this process, only a stamper 3 is mounted on a fixed mold 4 and forms pits and grooves on one side of the substrate 2 which is in direct contact with a movable mold 1. With this polycarbonate substrate, the tetralayer arrangement and the protective layer are formed for MOD and the bilayer arrangement for SD.
The substrate of the conventional a MOD or SD should undergo film-forming processes for the tetra- or bilayer arrangement, a spin coating process for the protective film and the bonding process for junction of two subunits. In the course of these processes, there is a strong possibility that the mechanical properties of the substrates may be degraded. This is highly apt to cause a problem in rotating the disk at a high rate which relates to data transfer rate.
The bonding of the two subunit disks can be usually achieved through use of a hot melt adhesive or UV setting resin. When a hot melt is used, a protective film made of hard coating resin, which plays a role in preventing flaws on the recording side of the polycarbonate substrate, is formed. In this case, the hot melt adhesive on the protective film may flow out and stick to the cartridge, giving rise to a serious problem in reliability. In the latter case, the uv setting resin is coated and exposed to be cured, which saves time. However, there occurs a problem in that, since the UV setting resin may flow out of the margin, it is required to be trimmed.